A photodetecting device which is capable of performing photodetection with a high sensitivity in a wide temperature range. A quantum dot structure including an embedding layer and quantum dots embedded by the embedding layer is formed. A quantum well structure including embedding layers and a quantum well layer whose band gap is smaller than those of the embedding layers is formed at a location downstream of the quantum dot structure in the direction of flow of electrons which flow perpendicularly to the quantum dot structure during operation of the photodetecting device. This reduces the temperature dependence of the potential barrier of a photodetecting section, which has to be overcome by electrons, whereby it is possible to lower the potential barrier of the embedding layers at high temperature.